Efforts have been directed toward the development of new analytical techniques needed in studies of glomerular capillary permeability in experimental animals, and toward the creation of mathematical models, based on fundamental membrane transport phenomena, needed for the interpretation of these physiological data. A fluorometric assay technique was developed which is suitable for determining protein concentration in nanoliter serum samples. This method, based on the reagent o-phthalaldehyde, provides an improved means for estimating colloid osmotic pressure in blood samples obtained by micropuncture from efferent arterioles. This quantity is needed for the determination of glomerular capillary permeability to water. Additional work has focused on electrostatic and electrokinetic influences on the permeation of macromolecules through the glomerular capillary wall, and their relation to proteinuria. A mathematical model of the charge-selectivity of the glomerular capillary wall is being developed, based on a modified Nernst-Planck equation. The model accounts for effects of membrane charge, molecular charge and size, and convection. To allow use of this model to interpret glomerular permeability data, it is necessary to measure effective molecular charge of the tracer molecules employed (principally dextran sulfate). Electrophoretic mobilities have been measured for tritiated dextran sulfates of varying molecular size, and effective molecular charge determined from these mobilities.